Computer-literate music fans who illegally share tracks over the internet also spend four and a half times as much on digital music as those who do not.
This doesn't really say much in the way of justifying file-sharing.
It does point out (and the article notes this) that people who share music are a valuable demographic for digital music sales. But it doesn't make any sort of case that file-sharing led them to purchase more music. It is entirely possible that if file-sharing didn't exist this same group would be buying 10 or 15 times as much music as the other group.
The survey confirms what many music fans have informally insisted for some time: that downloading tracks illegally has also led them to become more enthusiastic buyers of singles and albums online.
It does not confirm this. There is really no way of knowing how much digital music they would buy if they couldn't file-share.
Moreover, one would expect that "computer-literate" people would buy more digital music. The article doesn't really make clear whether this group buys more digital music than people who are computer literate and don't file-share or more than people who are not computer-literate. If it is the former, it can easily be explained that the other computer-literate group may not be music fans. If it is the latter, then it is simply a matter of being comfortable with technology.
Also, since it is only talking about digital music, we don't know how much non-digital music the non-file-sharing group is purchasing. It could be 30x as much.